Integrated circuits (ICs) often require selective one time programmable (OTP) permanent electrical connections between circuit nodes. Such a connection can be implemented by an antifuse. Antifuses are often used to permanently store binary data on an IC. Binary logic states are represented by "on" and "off" states of the antifuse. Antifuses are used in numerous memory storage applications including programmable logic arrays (PALs), programmable logic devices, and programmable read only memories (PROMs). Antifuses are also often used in memory cell arrays such as dynamic random access memories (DRAMs). After testing the DRAM for failing memory cells, failing cell addresses in the DRAM are remapped to functional cell addresses by selective permanent programming of antifuse elements.
Antifuses are fabricated with structure similar to that of a capacitor; two conductive electrical terminals are separated by a dielectric layer. An unprogrammed "off" state, in which the antifuse is fabricated, presents a high resistance between the antifuse terminals. The antifuse can also be programmed to an "on" state in which a low resistance connection between the antifuse terminals is desired. To program an antifuse "on," a large programming voltage is applied across the antifuse terminals, breaking down the interposed dielectric and forming a conductive link between the antifuse terminals.
For a programmed antifuse in the "on" state, a low resistance conductive link between the antifuse terminals is desired in order to minimize access time in reading the state of the antifuse. A characteristic RC time constant is formed by the resistance of the antifuse and the input capacitance of antifuse detection circuitry. By minimizing the resistance of the antifuse when programmed "on", access time for reading the state of the antifuse is reduced. A low "on" state resistance also obtains a better noise margin for properly detecting the state of the antifuse. In addition, lower initial resistance ensures stable and robust reading of the antifuse in spite of a gradual increase in resistance over time due to continued oxidation of the antifuse.
However, the conductive link between the antifuse terminals is sometimes only marginally conductive due to, for example, variations in dielectric thickness between antifuses. Such variations in dielectric thickness between antifuses may occur on the same integrated circuit, on different integrated circuits produced by the same fabrication process, or on different integrated circuits produced by different fabrications processes. Marginally conductive antifuses typically have resistances which are not well controlled, increasing the difficulty in accurately reading the state of the antifuse by detection circuitry.
For marginally conductive antifuses, it is desirable to apply the large programming voltage for a longer period of time or to apply multiple programming pulses in order to further reduce the resistance across the antifuse and thereby avoid only marginally programming an antifuse into its "on" state. One example of using multiple programming pulses to lower the resistance of a programmed antifuse is found in U.S. Pat. No. 5,257,222, which issued on Oct. 26, 1993 entitled "ANTIFUSE PROGRAMMING BY TRANSISTOR SNAP-BACK", which is assigned to the assignee of the present invention.
It is also desirable to restrict use of extended programming voltages only to marginally programmed antifuses, since an adequately programmed antifuse draws a large current. Such large currents through the adequately programmed antifuse may degrade the antifuse dielectric or create electromigration damage to associated metal interconnections. There is a need to reduce the resistance across a programmed antifuse without degrading the antifuse dielectric or creating damage to metal interconnections.